The present invention relates to a spread spectrum communication system and a method for the same in which spread spectrum modulation using a direct spread (DS) method is carried out by using the same spread code list for a parallel data list, spread spectrum modulation signals are multiplexed and transmitted, and the transmitted spread spectrum modulation signals are demodulated to receive the parallel data list.
In recent years, Code Division Multiple Access (CDMA) communicating method using a spread spectrum method as one of traffic transmitting methods such as images, voices, data and the like have been put into practical use in a mobile communication system and a satellite communication system.
The spread spectrum communicating method includes a direct spread (DS) method, a frequency hopping (FH) method and the like. In the DS method, a spread code list having a much wider band than the band of an information signal is directly multiplied by the information signal, thereby performing the spread spectrum of the information signal to carry out communication.
FIG. 17 is a block diagram showing the structure of a conventional spread spectrum communication system in which spread spectrum is carried out by using the same spread code list for parallel data, different time delays are given to spectrum spread signals to be multiplexed and transmitted. In FIG. 17, a data generating section 101 generates a binary digital information signal having a value of xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d. In the following description, the speed at which the digital information signal is generated will be referred to as a bit rate and it is denoted as xe2x80x9cRbxe2x80x9d. A serial/parallel converting section 102 converts the digital information signal input from the data generating section 101 into parallel information signal having n channels. A multiplex number n is equal to or less than spread code length L (bit). In the following description, the speed at which the parallel information signal in each channel is generated will be referred to as a parallel bit rate and it is denoted as xe2x80x9cRpxe2x80x9d (=Rb/n).
A spread code generating section 104 generates a spread code list. The spread code list has a value of a xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d and a spread code list having a spread code length L (bit) which is generated by a clock generating section 103 and has a clock frequency band that is spread code length L times as much as the parallel bit rate Rp. It is desirable that the circuit used for creating the codes has a simple structure and that the codes have great auto-correlation characteristics and small cross-correlation therebetween. Therefore, as a proper code list, for example, M list, Gold code and the like are used. In the following description, a clock rate generated by the clock generating section 103 will be referred to as a chip rate Rc (=Rpxc3x97L) and a clock cycle having the chip rate Rc will be referred to as a chip cycle Tc (=1/Rc).
Each one of the n channel parallel information signal converted by the serial/parallel converting section 102 is input into the spread modulating sections 105-1 to 105-n, respectively. The spread modulating sections 105-1 to 105-n multiply the parallel information signal and a spread code input from the spread code generating section 104, thereby generating parallel spectrum spread signals for n channels. As a result, each of the parallel spectrum spread signal has the chip rate Rc.
Each parallel spectrum spread signal is input into each of the delay sections 107-1 to 107-n. Each of the delay sections 107-1 to 107-n delays the parallel spectrum spread signal by times {b1, b2, b3, . . . bn} respectively and output the same signals to a multiplexing section 108. The multiplexing section 108 performs multiplexing by adding each of the parallel spectrum spread signal which have been delayed differently, and a multiplexed spectrum spread signal thus obtained is sent to a frequency converting section 109. Although each of the parallel spectrum spread signal is spectrum-spread by the same spread code list in the spread modulating sections 105-1 to 105-n, they have been delayed differently in the delay sections 107-1 to 107-n. Therefore, a small cross-correlation is obtained between data lists at the time of the code synchronization of the data list corresponding to each of the parallel spectrum spread signal on the receiving side for receiving the multiplexed parallel spectrum spread signal.
A frequency converting section 109 frequency-converts the input multiplexed spectrum spread signal to obtain the radio frequency (RF), then generates a multiplexed RF signal is power-amplified by a power amplifying section 110 and transmitted using an antenna 111.
FIG. 18 is a block diagram showing the structure of a conventional spread spectrum communication system for receiving the multiplexed RF signal. In FIG. 18, an RF amplifying section 122 receives the multiplexed RF signal from an antenna 121 and amplifies the multiplexed RF signal. A quadrature detecting section 123 causes a multiplier 141 to multiply a local carrier signal output from a voltage-controlled oscillator (VCO) 143 and the multiplexed RF signal input from the RF amplifying section 122, causes a low-pass filter 145 to remove the high-frequency component of the multiplied signal, and furthermore, causes an A/D converter 147 to convert the multiplied signal into digital data, thereby generating the in-phase component of the complex spectrum spread signal having the frequency band of the chip rate Rc. On the other hand, a multiplier 142 also multiplies the local carrier signal which is phase-shifted by xcfx80/2 by means of a xcfx80/2 phase shifter 144 and the multiplexed RF signal input from the RF amplifying section 122 for a signal output from the voltage-controlled oscillator 143, a low-pass filter 146 removes the high-frequency component of the multiplied signal, and furthermore, an A/D converter 148 converts the multiplied signal into digital data, thereby generating the orthogonal component of the complex spectrum spread signal having a frequency band of a chip rate Rc.
A correlation value calculating section 124 obtains a correlation between the complex spectrum spread signal input from the quadrature detecting section 123 and a spread code list which is the same as the spread code list generated by the spread code generating section 104. More specifically, an in-phase correlation value calculating section 149 in the correlation value calculating section 124 outputs, as an in-phase correlation value, a correlation value of the complex spectrum spread signal and the spread code list which is the same as a spread code multiplied to the multiplexed RF signal. Further, an orthogonal correlation value calculating section 150 in the correlation value calculating section 124 outputs, as an orthogonal correlation value, a correlation value of the complex spectrum spread signal and the spread code list which is the same as the spread code multiplied to the multiplexed RF signal. The correlation in the in-phase correlation value calculating section 149 and the orthogonal correlation value calculating section 150 can be implemented by using a matched filter and the like.
A code synchronizing section 125 generates a symbol clock CK synchronized with the cycle of the spread code list multiplied by the multiplexed RF signal from the in-phase correlation value and the orthogonal correlation value which are output from the correlation value calculating section 124, and outputs the symbol clock CK to data demodulating sections 128-1 to 128-n.
Delay correcting sections 126-1 to 126-n perform delay correction in such a manner that the timings of the peak values of the in-phase correlation value and the orthogonal correlation value for corresponding parallel spectrum spread signals match respectively. More specifically, the shift of the timings of the parallel spectrum spread signals which is caused due to the provision of the delay by the delay sections 107-1 to 107-n shown in FIG. 17 is eliminated.
The data demodulating sections 128-1 to 128-n latch the in-phase correlation value and the orthogonal correlation value in a timing synchronous with the symbol clock CK for the in-phase correlation values and the orthogonal correlation values for n channels which are output from the delay correcting sections 126-1 to 126-n, respectively. Based on the latched in-phase correlation value and orthogonal correlation value, data decision is carried out to perform demodulation. Then, the demodulation data output from each of the data demodulating sections 128-1 to 128-n are converted into serial data by a parallel/serial converting section 129 and are output as n channel demodulation data. The parallel demodulation data output from the data demodulating sections 128-1 to 128-n have a parallel bit rate Rp and are binary data signals having a value of xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d. Accordingly, the demodulation data having 1-channel bit rate Rb (=nxc3x97Rp) are output from the parallel/serial converting section 129.
The above-mentioned multiplexed RF signal is generated by multiplexing the n channel parallel spectrum spread signals multiplied by the same spread code list. However, the cross-correlation between the channels during the data demodulation is reduced by giving various delay times for the channels. Therefore, it is possible to perform the demodulation for the channels by the data demodulating sections 128-1 to 128-n.
In the conventional spread spectrum communication system, thus, the n channel parallel information signal can be transmitted and received by using one spread code list. In the spread spectrum communication system on the receiving side, therefore, the n channel parallel information signal can be received by using only a pair of correlators. Thus, a simple structure is sufficient. More specifically, in a general spectrum spread communicating method, in the case where a RF signal having the n channel multiplexed is to be transmitted and received, n different spread code lists are assigned to each channel to perform the spectrum spread for multiplexing and the demodulation is carried out by using n correlators having n different spread code lists on the receiving side. In the conventional spread spectrum communication system, however, the multiplexed RF signal can be generated by one spread code list and the signals for n channels can be demodulated by using only a pair of correlators.
In the conventional spread spectrum communication system described above, all the n channel parallel spectrum spread signals are added in the multiplexing section 108 to generate a multiple spectrum spread signal, and a frequency converting section 109 converts the generated multiplexed spectrum spread signal into the multiplexed RF signal. Therefore, the maximum amplitude of the multiplexed spectrum spread signal is n times as much as the amplitude of each of the parallel spectrum spread signal. The multiplexed RF signal has a maximum value which is n times as much as the maximum value of the RF signal without multiplexing.
In general, the multiplexed spectrum spread signal is a multivalued modulation signal. In the case where the amplitude of the multivalued modulation signal is to be power-amplified, it is desirable that the power amplifying section 110 should be used on an operating point which is as close to a saturation point as possible, that is, an operating point having a small output back-off in order to enhance the power efficiency.
However, in the case where the operating point of the amplifier is set in a region having a small output back-off, non-linearity such as input amplitude/output amplitude non-linear (AM/AM) characteristic and the like is notable. For example, in the case where the power amplifying section 110 shown in FIG. 17 is to be used on an operating point having a small output back-off, there has been a problem that the output signal of the power amplifying section 110 is greatly distorted to deteriorate a bit error ratio during transmission and receipt in the spread spectrum communication system if a signal having a great maximum amplitude is input, to the power amplifying section 110, as the RF signal output from the frequency converting section 109.
It is an object of the present invention to obtain a spread spectrum communication system and a method for the same which can reduce a deterioration in the bit error ratio of each information multiplexed into a multiplexed spectrum spread signal also in the case where an operation is carried out on an operating point having the small output back-off of a power amplifier.
According to the spread spectrum communication system of the present invention, a converting unit converts input serial information into parallel information, a spread code generating unit generates a spread code list synchronous with a clock generated by the clock generating unit for generating a clock having a frequency band of a chip rate corresponding to a spread code length, a spread modulating unit multiplies each of the parallel information list converted by the converting unit and the spread code list generated by the spread code generating unit respectively, thereby generating a plurality of parallel spectrum spread signals corresponding to the parallel information list, a phase shifting unit shifts the parallel spectrum spread signals by a plurality of phase shifting amounts, a delay unit delays the parallel spectrum spread signals having the phases shifted by the phase shifting unit by different amounts, a multiplexing unit generates a multiplexed spectrum spread signal obtained by adding the parallel spectrum spread signals delayed by the delay unit, and a transmitting unit converts the multiplexed spectrum spread signal generated by the multiplexing unit into a radio frequency signal, amplifies and transmits the radio frequency signal. Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced.
According to the spread spectrum communication system of the present invention, a receiving unit receives a multiplexed spectrum spread signal which is obtained by adding and multiplexing a plurality of spectrum spread signals which are spread in a predetermined spread code list, each of the spectrum spread signals having a plurality of phase shifting amounts and a plurality of different delay amounts respectively, a quadrature detecting unit generates, from the multiplexed spectrum spread signal, in-phase components and orthogonal components of the spectrum spread signals having a frequency band of a chip rate corresponding to the predetermined spread code list, a correlating unit calculates an in-phase correlation value and an orthogonal correlation value which are correlation values between the in-phase components and the orthogonal components of the spectrum spread signals and the predetermined spread code list, a code synchronizing unit generates a symbol clock which is common to the spectrum spread signals based on the in-phase correlation value and the orthogonal correlation value, a delay correcting unit branches the in-phase correlation value and the orthogonal correlation value corresponding to a plurality of values of the spectral spread signals and performs delay correction to arrange peak values of the in-phase correlation value and the orthogonal correlation value corresponding to each delay amount of the spectrum spread signals, a phase-shift correcting unit corrects shift of carrier phases of the in-phase correlation value and the orthogonal correlation value which are delay corrected by the delay correcting unit corresponding to the phase shifting amounts, a demodulating unit generates a plurality of demodulating signals corresponding to the spectrum spread signal from each of the in-phase correlation values and orthogonal correlation values which have the phases shifted by the phase-shift correcting unit, and a converting unit converts a plurality of demodulation signals demodulated by the demodulating unit into serial information. Consequently, each of the parallel spectrum spread signal can reliably be demodulated based on the multiplexed spectrum spread signal having a small maximum amplitude.
According to the spread spectrum communication system of the present invention, when carrying out parallel communication of a plurality of channels by using the same spread code list between a transmitting side device and a receiving side device, in the transmitting side device, a first converting unit converts input serial information into parallel information, a spread code generating unit generates a spread code list synchronous with the clock generated by the clock generating unit for generating a clock having a frequency band of a chip rate corresponding to a spread code length, a spread modulating unit multiplies each of the parallel information list converted by the first converting unit and the spread code list generated by the spread code generating unit respectively, thereby generating a plurality of parallel spectrum spread signals corresponding to the parallel information list, a phase shifting unit shifts the parallel spectrum spread signals by a plurality of phase shifting amounts, the delay unit delays the parallel spectrum spread signals having the phases shifted by the phase shifting unit by different amounts, a multiplexing unit generates a multiplexed spectrum spread signal obtained by adding the parallel spectrum spread signals delayed by the delay unit, and a transmitting unit converts the multiplexed spectrum spread signal generated by the multiplexing unit into a radio frequency signal, amplifies and transmits the radio frequency signal, and in the receiving side device, a receiving unit receives a multiplexed spectrum spread signal transmitted from the transmitting unit, a quadrature detecting unit generates, from the multiplexed spectrum spread signal, in-phase components and orthogonal components of the spectrum spread signals having a frequency band of a chip rate corresponding to the same spread code list, a correlating unit calculates an in-phase correlation value and an orthogonal correlation value which are correlation values between the in-phase components and the orthogonal components of the spectrum spread signals and the predetermined spread code list, a code synchronizing unit generates a symbol clock which is common to the spectrum spread signals based on the in-phase correlation value and the orthogonal correlation value, a delay correcting unit branches the in-phase correlation value and the orthogonal correlation value corresponding to a plurality of values of the spectral spread signals and performs delay correction to arrange peak values of the in-phase correlation value and the orthogonal correlation value corresponding to each delay amount of the spectrum spread signals, a phase-shift correcting unit corrects shift of carrier phases of the in-phase correlation value and the orthogonal correlation value which are delay corrected by the delay correcting unit corresponding to the phase shifting amounts, a demodulating unit generates a plurality of demodulating signals corresponding to the spectrum spread signal from each of the in-phase correlation values and orthogonal correlation values which have the phases shifted by the phase-shift correcting unit, and a second converting unit converts a plurality of demodulation signals demodulated by the demodulating unit into serial information. Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced.
Further, a part of the phase shifting amounts to be phase-shifted by the phase shifting unit is set to an arbitrary phase of xcex2 or a phase of (xcex2+xcfx80) and the other phase shifting amounts are set to a phase of (xcex2+xcfx80/2) or a phase of (xcex2xe2x88x92xcfx80/2). Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced by the minimum change in the phase.
Further, the phase shifting amounts to be phase-shifted by the phase shifting unit is n (n is a natural number which is equal to or greater than 2), each phase shifting amount being an arbitrary phase of (xcex3+pxcfx80/n) or a phase of (xcex3+pxcfx80/n+xcfx80) (p=0 to nxe2x88x921). Consequently, the phase shifting amount is given in such a manner that the maximum amplitude of the multiplexed spectrum spread signal can be minimized.
According to the spread spectrum communication method of the present invention, a converting step converts input serial information into parallel information, the spread modulating step multiplies each of the parallel information list converted at the converting step and a predetermined spread code list, thereby generating a plurality of parallel spectrum spread signals corresponding to the parallel information list, a phase shifting step shifts the parallel spectrum spread signals by a plurality of phase shifting amounts, a delay step delays the parallel spectrum spread signals having the phases shifted by the phase shifting step by different amounts, a multiplexing step generates a multiplexed spectrum spread signal obtained by adding the parallel spectrum spread signals delayed at the delay step, and a transmitting step converts the multiplexed spectrum spread signal generated at the multiplexing step into a radio frequency signal and amplifies and transmits the radio frequency signal. Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced.
According to the spread spectrum communication method of the present invention, a receiving step receives a multiplexed spectrum spread signal which is obtained by adding and multiplexing a plurality of spectrum spread signals which are spread in a predetermined spread code list, each of the spectrum spread signals having a plurality of phase shifting amounts and a plurality of different delay amounts respectively, a quadrature detecting step generates, from the multiplexed spectrum spread signal, in-phase components and orthogonal components of the spectrum spread signals having a frequency band of a chip rate corresponding to the predetermined spread code list, a correlating step calculates an in-phase correlation value and an orthogonal correlation value which are correlation values between the in-phase components and the orthogonal components of the spectrum spread signals and the predetermined spread code list, a code synchronizing step generates a symbol clock which is common to the spectrum spread signals based on the in-phase correlation value and the orthogonal correlation value, a delay correcting step branches the in-phase correlation value and the orthogonal correlation value corresponding to a plurality of values of the spectral spread signals and performs delay correction to arrange peak values of the in-phase correlation value and the orthogonal correlation value corresponding to each delay amount of the spectrum spread signals, a phase-shift correcting step corrects shift of carrier phases of the in-phase correlation value and the orthogonal correlation value which are delay corrected at the delay correcting step corresponding to the phase shifting amounts, a demodulating step generates a plurality of demodulating signals corresponding to the spectrum spread signals from each of the in-phase correlation values and orthogonal correlation values which have the phases shifted at the phase-shift correcting step by using the clock symbol clock generated at the code synchronizing step, and a converting step converts a plurality of demodulation signals demodulated at the demodulating step into serial information. Consequently, each of the parallel spectrum spread signal can reliably be demodulated based on the multiplexed spectrum spread signal having a small maximum amplitude.
According to the spread spectrum communication method of the present invention, when carrying out parallel communication of a plurality of channels by using the same spread code list between a transmitting side device and a receiving side device, a first converting step converts input serial information into parallel information, a spread modulating step multiplies each of the parallel information list converted at the first converting step and the same spread code list respectively, thereby generating a plurality of parallel spectrum spread signals corresponding to the parallel information list, a phase shifting step shifts the parallel spectrum spread signals by a plurality of phase shifting amounts, the delay step delays the parallel spectrum spread signals having the phases shifted at the phase shifting step by different amounts, a multiplexing step generates a multiplexed spectrum spread signal obtained by adding the parallel spectrum spread signals delayed at the delay step, a transmitting step converts the multiplexed spectrum spread signal generated at the multiplexing step into a radio frequency signal and amplifies and transmits the radio frequency signal, and in the receiving side, a receiving step receives the multiplexed spectrum spread signal transmitted at the transmitting step, a quadrature detecting step generates, from the multiplexed spectrum spread signal, in-phase components and orthogonal components of the spectrum spread signals having a frequency band of a chip rate corresponding to the same spread code list, a correlating step calculates an in-phase correlation value and an orthogonal correlation value which are correlation values between the in-phase components and the orthogonal components of the spectrum spread signals and the predetermined spread code list, a code synchronizing step generates a symbol clock which is common to the spectrum spread signals based on the in-phase correlation value and the orthogonal correlation value, a delay correcting step branches the in-phase correlation value and the orthogonal correlation value corresponding to a plurality of values of the spectral spread signals and carries out delay and correction to arrange peak values of the in-phase correlation value and the orthogonal correlation value corresponding to each delay amount of the spectrum spread signals, a phase-shift correcting step corrects shift of carrier phases of the in-phase correlation value and the orthogonal correlation value which are delay corrected at the delay correcting step corresponding to the phase shifting amounts, a demodulating step generates a plurality of demodulating signals corresponding to the spectrum spread signals from each of the in-phase correlation values and orthogonal correlation values which have the phases shifted at the phase-shift correcting step by using the symbol clock generated at the code synchronizing step, and a second converting step converts a plurality of demodulation signals demodulated at the demodulating step into serial information. Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced.
Further, a part of the phase shifting amounts to be phase-shifted at the phase shifting step is set to an arbitrary phase of xcex2 or a phase of (xcex2+xcfx80) and the other phase shifting amounts are set to a phase of (xcex2+xcfx80/2) or a phase of (xcex2xe2x88x92xcfx80/2). Consequently, the maximum amplitude of the multiplexed spectrum spread signal can be reduced by the minimum change in the phase.
Further, the phase shifting amounts to be phase-shifted at the phase shifting step is n (n is a natural number which is equal to or greater than 2), each phase shifting amount being an arbitrary phase of (xcex3+pxcfx80/n) or a phase of (xcex3+pxcfx80/n+xcfx80) (p=0 to nxe2x88x921). Consequently, the phase shifting amount is given in such a manner that the maximum amplitude of the multiplexed spectrum spread signal can be minimized.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.